1. Field of the Invention
The present invention relates to a circuit for controlling a Light Emitting Diode (LED) which is employed in a backlight system or a lighting system. More particularly, the present invention relates to a circuit for controlling an LED which can linearly control luminance and color according to changes in an ambient temperature to more precisely compensate for temperature-induced variations in LED properties, and save the cost of the product due to no requirement of a microprocessor.
2. Description of the Related Art
In general, a Cold Cathode Fluorescent Lamp (CCFL) is largely employed in a Liquid Crystal Display (LCD) and other back light systems for electronic display. However, attempts have been made to substitute a light emitting diode (LED) for the CCFL in the backlight system for various reasons. That is, with the LED employed, a color gamut is expanded and a white point can be controlled through color control. Also, advantageously, the LED is devoid of mercury and thus environment-friendly.
The LED backlight system combines red (R), green (G) and blue (B) light into white light to use as a light source. The R, G, B LEDs for use in the backlight system vary in their properties depending on a voltage applied, ambient temperature and operation time. Also, the R, G and B LEDs differ in their own characteristics considerably.
Accordingly, in the LED-based backlight system or all systems using the LED as a light source, it is necessary to control luminance and color to be uniform regardless of environmental changes such as ambient temperature, aging effects of the LED and differences in LED properties.
FIG. 1 is a block diagram illustrating a conventional light emitting control device.
Referring to FIG. 1, the conventional light emitting device 10 detects a forward voltage Vf of an LED device 1, estimates an ambient temperature Ta from the detected forward voltage Vf, derives an optimal feedback point of a driving current of the LED device 1 and controls a light emitting amount of the LED device 1.
The conventional light emitting control device 10 includes an A/D converter 12, a feedback point decider 14, a temperature properties memory 16, a PWM controller 27 and a PWM circuit 28. The A/D converter 12 detects the forward voltage Vf of the LED device 1 and converts it into a digital signal. The feedback point decider 14 estimates the ambient temperature Ta of the LED device 1 via the forward voltage Vf from the A/D converter 12 and decides the optimum feedback point of the driving current of the LED device 1 based on the ambient temperature Ta. The temperature properties memory 16 memorizes a Vf-Ta table 17 for correlating the forward voltage Vf of the LED device 1 with the ambient temperature Ta and a Ta-Ifmax table 19 for correlating the ambient temperature Ta with a maximum allowable current Ifmax. The PWM controller 27 performs PWM control of the LED device 1 in response to decision by the feedback point decider 14. The PWM circuit 28 drives the LED device by PWM under the control of the PWM controller 27.
Here, the Vf-Ta table 17 and Ta-Ifmax table 19 are preset based on temperature properties of the LED device 1 described later. The feedback point decider 14 refers to a table of the temperature properties of the LED device 1 memorized by the temperature properties memory 16 to decide the ambient temperature Ta and the driving current.
Furthermore, temperature properties of the LED device 1 vary with the types of the LED device 1. Accordingly the Vf-Ta table 17 and the Ta-Ifmax table 19 are specified by the type of the LED device 1.
A temperature calculator 13 of the feedback point decider 14 refers to the Vf-Ta table 17 memorized by the temperature properties memory 16 to derive the ambient temperature Ta via the detected forward voltage Vf. The driving current decider 15 of the feedback point decider 14 decides the feedback point of the driving current of the LED device 1 and then a control value of the driving current so that the ambient temperature Ta calculated by the temperature calculator 13 falls within a range of an ambient temperature for driving the LED device 1 and a desired light emitting amount of the LED device 1 is achieved.
For example, in a case where the ambient temperature Ta calculated by the temperature calculator 13 is lower than an upper limit of an ambient temperature for driving the LED device 1 and thus luminance of the LED device 1 needs to be further increased, the driving current decider 15 decides the control value so that the driving current is raised. Also, in a case where the ambient temperature Ta approximates an upper limit of an ambient temperature for driving, the driving current decider 15 decides the control value so that the driving current is reduced.
That is, the forward voltage of the LED device 1 is measured according to changes in temperature and current temperature is estimated based on a pre-memorized temperature vs. forward voltage table. Then a maximum allowable current of the LED device 1 is adjusted via a table of the maximum allowable current according to temperature to control the driving voltage of the LED device 1.
However, such a conventional method needs to employ a microprocessor to ensure more precise control, disadvantageously increasing production costs.
FIG. 2 is a configuration diagram illustrating a conventional backlight device.
The conventional backlight device of FIG. 2 includes a power supply 110, light sources 150 and 160, a temperature sensor 250, photo diodes 210 and a controller 180. The power supply 110 is comprised of a plurality of LED drivers 120 to 140 for driving by an alternating current 115. The light sources 150 and 160 are comprised of a plurality of LEDs which are turned on by the drivers 120 to 140 of the power supply 110 to emit light, and supply light into a light guide 170. The temperature sensor 250 senses temperature of the light sources 150 and 160. The photo diodes 210 are disposed in the middle of both sides of the light guide 170 to sense luminance of light. The controller 180 compensates for temperature-related variations in luminance and color based on temperature measured by the temperature sensor 250 through an interface for detection 230 and luminance determined by the photo diode 210.
The conventional backlight device employs both the temperature sensor and the photo sensor. Here, in order to control the LED driver, temperature is measured via the temperature sensor and a light amount of the LED device is measured via the photo sensor to maintain a desired light amount. Such a control is enabled via a microprocessor.
In this case, the respective light amount of R, G and B LEDs is measured through photo sensors equipped with a filter. With the values measured, the R, G and B LEDs are controlled respectively so as to maintain the light amount which is perceived and targeted by the microprocessor. Also, temperature is measured via the temperature sensor attached to a heat sink to compensate for variations in LED properties according to the measured temperature.
However, like the conventional method of FIG. 1, this conventional method of FIG. 2 is disadvantageous in terms of manufacturing costs for the system.